Implantable neurostimulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal Cord Stimulation (SCS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of tissue stimulation has begun to expand to additional applications such as angina pectoralis and incontinence. Deep Brain Stimulation (DBS) has also been applied therapeutically for well over a decade for the treatment of refractory chronic pain syndromes, and DBS has also recently been applied in additional areas such as movement disorders and epilepsy. Further, in recent investigations Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. Furthermore, Functional Electrical Stimulation (FES) systems such as the Freehand system by NeuroControl (Cleveland, Ohio) have been applied to restore some functionality to paralyzed extremities in spinal cord injury patients.
Each of these implantable neurostimulation systems typically includes at least one stimulation lead implanted at the desired stimulation site and an implantable Pulse Generator (IPG) implanted remotely from the stimulation site, but coupled either directly to the stimulation lead(s) or indirectly to the stimulation lead(s) via one or more lead extensions. Thus, electrical pulses can be delivered from the neurostimulator to the electrodes carried by the stimulation lead(s) to stimulate or activate a volume of tissue in accordance with a set of stimulation parameters and provide the desired efficacious therapy to the patient. A typical stimulation parameter set may include the electrodes that are sourcing (anodes) or returning (cathodes) the stimulation current at any given time, as well as the amplitude, duration, rate, and burst rate of the electrical pulses. Significant to the present inventions described herein, a typical IPG may be manually inactivated by the patient by placing a magnet over the implanted IPG, which closes a reed switch contained within the IPG.
The neurostimulation system may further comprise a handheld Remote Control (RC) to remotely instruct the neurostimulator to generate electrical pulses in accordance with selected stimulation parameters. The RC may, itself, be programmed by a technician attending the patient, for example, by using a Clinician's Programmer (CP), which typically includes a general purpose computer, such as a laptop, with a programming software package installed thereon.
Electrical stimulation energy may be delivered from the neurostimulator to the electrodes using one or more current-controlled sources for providing electrical pulses of a specified and known current (i.e., current regulated output pulses), or one or more voltage-controlled sources for providing electrical pulses of a specified and known voltage (i.e., voltage regulated output pulses). The circuitry of the neurostimulator may also include voltage converters, power regulators, output coupling capacitors, and other elements as needed to produce constant voltage or constant current stimulus pulses.
Neurostimulation systems, which may not be limited to SCS used to treat chronic pain, are routinely implanted in patients who are in need of Magnetic Resonance Imaging (MRI). Thus, when designing implantable neurostimulation systems, consideration must be given to the possibility that the patient in which neurostimulator is implanted may be subjected to electro-magnetic interference generated by MRI scanners, which may potentially cause damage to the neurostimulator as well as discomfort to the patient. Typically, electromagnetic interference (EMI) filters are placed between the ports of the neurostimulators and the tissue contacting electrodes to ensure correct operation and prevent damage to the neurostimulator in environments with high radio frequency (RF) fields, such as MRI-environments. Typically, these filters incorporate capacitors, which shunt the RF energy to a common node, such as the metallic case of the neurostimulator to protect internal circuitry. Additionally, lead inter-conductor capacitances contribute shunting capacitances between the neurostimulator outputs. Another source of shunting capacitance is the capacitance of internal neurostimulator circuit elements, such as active electronic switches disposed for the delivery of electrical pulses.
During the delivery of electrical pulses, the shunting capacitances must be charged to the voltage present on the electrode during the delivery of the electrical pulses. The charge absorbed or delivered from the capacitances alters the wave shape of the delivered electrical pulses on the active electrodes.
For example, normally the neurostimulator drives current into tissue at the electrodes at the end of the neurostimulation leads. Each electrode can be treated as an ideal current source, with the tissue approximated as a resistor network. The total current delivered to the tissue by the electrodes must be equal to zero (i.e., the total magnitude of the anodic current must be equal to the total magnitude of the cathodic current). For example, as illustrated in FIG. 1, the simplest case is an electrical source (in this case, a current source I) between two electrodes, one acting as an anode E1, and the other acting as a cathode (in this case, a case electrode Ecase). Of course, electrode E1 can alternatively be the cathode E1, and the case electrode Ecase may be the anode. As such, the current flow shown by the arrow of the electrical source illustrated in FIG. 1, as well as the subsequent figures provided in this specification, is arbitrary, and thus, the direction of the arrow represented in the current sources provided herein does not necessarily mean that the current actually flows in that direction.
Thus, electrical current it, which is equal to the current generated by the current source I, flows through the tissue resistance Rt between the electrode E1 and the case electrode Ecase (i.e., anodic electrical current enters the tissue from electrode E1 and an cathodic electrical current of equal magnitude exits the tissue into the case electrode Ecase). The voltage V developed across the tissue resistance Rt is described by ohm's law (V=IR). It should be noted that the true circuit includes DC blocking capacitors. However, these DC blocking capacitors are sufficiently large to have negligible voltage change during delivery of an electrical pulse, they are normally ignored for the purposes of analyzing the electrical properties of the stimulation energy, and so are not explicitly shown in the simplified model illustrated in FIG. 1.
However, as discussed above, it is possible for shunt capacitances, such as EMI filters, internal stimulation circuitry capacitances, and lead inter-conductor capacitances, to have parasitic components that do distort the stimulation enough to be of concern. An example of such a circuit is illustrated in FIG. 2, where C1, R1, and C2 represent lumped element models of the parasitic components of the neurostimulation lead, and any shunt capacitances in EMI filters and internal stimulation circuitry between two electrodes, one acting as a cathode E1, and the other acting as an anode (in this case, a case electrode Ecase). Current i1 and i2 will leak through the shunt capacitances C1 and C2 in response to changes in the voltage across these capacitances, preventing the specified electrical drive current I from reaching the tissue, modeled as Rt. Instead, current it flows through the tissue resistance Rt.
This shunting phenomenon alters the shape of each electrical pulse, and thus, the waveshape of the current delivered to the tissue of the patient. In the case where the stimulation source approximates a voltage source, the total charge delivered to the patient may also be altered. For example, the rise time of the electrical pulse current may be increased while the capacitances connected to the active electrodes are charging. For example, as shown in FIG. 3, the ideal electrical pulse current has a relatively short rise time, whereas the actual electrical pulse current due to the absorption of charge from the capacitances has a relatively long rise time. Although the ideal electrical pulse is shown as being square, in reality, the electrical pulse current will have a nominally trapezoidal wave shape, with the current flowing in the shunting capacitances during the rising and falling edges of the pulse.
In either event, this will result in an unintended change in the total charge delivered to the tissue during the electrical pulse. Further, inactive electrodes may also be subject to voltage shifts during the delivery of the electrical pulse due to their contact with tissue near the stimulation site, resulting in changes in the charge level of the capacitors connected to these electrodes. The change in charge may result in unintended delivery or removal of charge at the associated tissue-contacting electrode and unintended changes in polarization of this tissue, or even unintended tissue stimulation.
There, thus, remains a need to compensate for changes in charge at tissue-contacting electrodes due to shunt capacitances in the stimulation circuit.